Various metering devices are already known which have in common the characteristic whereby they include two compartments, namely a first, metering compartment filled with a predetermined volume of liquid in a first, inverted position of the container, and a second, reserve compartment communicating with the metering compartment so as to receive the predetermined volume of liquid contained in the metering compartment when the container is returned to its normal position. The reserve compartment furthermore communicates with an outlet of the metering device so as to pour out a predetermined measure of liquid which has previously been withdrawn and measured as indicated above. Pouring-out of the predetermined measure takes place simultaneously with the filling of the metering compartment.
A process is thus known withdrawal and pouring out of a predetermined measure of liquid from a said liquid contained in a container, the process comprising at least one sequence whereby, from a normal position of the said container, in which the said container rests on its base, successively the container is inverted, top to bottom, and then the said container is returned to the normal position, this being in order to proceed to the stages required for withdrawal and metering of the said liquid, in which:                (a) from the contents of the container, a predetermined amount of liquid, equal to the sum of the measure of the said liquid to be poured out plus a surplus of the said liquid is withdrawn and isolated from the said container, in a withdrawal chamber;        (b) from the predetermined amount of liquid, a metering chamber is filled by transfer from the withdrawal chamber, in order to obtain the said measure;        (c) and the said measure is poured out from the metering chamber.        
It is known to produce a metering device for implementing the above mentioned process, comprising two coaxial elements, one which is tubular or internal, and the other which is cylindrical or external, together defining a withdrawal chamber and a metering chamber arranged one above the other in a reference “normal” position of the said device and communicating with one another at a level located between the two said chambers with the aid of a communication or transfer opening, the said withdrawal chamber communicating with the outside, for example the inside of a container at a low level and at an intermediate level, determining between them, in the withdrawal chamber and in the inverted position of the said device, a predetermined withdrawal volume which is by construction greater than the volume of the metering chamber.
By way of example of a device of this type, reference will made, for example, to document FR-A-2 590 555.
Document U.S. Pat. No. 5,148,953 or document GB-2 145 061 also disclose a process which operates according to the process described hereinabove.
In accordance with document U.S. Pat. No. 5,148,953, by virtue of one or a number of longitudinal slots (i.e. slots parallel to the axis of the device) provided in that part of the outer wall of the withdrawal chamber which extends from the level of the hole for communication between the two, withdrawal and metering, chambers, during or at the end of the stage of transfer from the withdrawal chamber to the metering chamber, the surplus of the liquid withdrawn relative to the amount strictly required for metering is evacuated outside the metering device and returns to the inside of the container. Thus, during the pouring-out stage, only the metered amount is evacuated to the outside of the metering device.
In accordance with document GB-2 145 061, at the end of the stage of transfer from the withdrawal chamber to the metering chamber, by virtue of an overflow means returning the surplus of the liquid withdrawn towards the outside of the metering device, i.e. into the container, a common level of the liquid, corresponding to that of the metered liquid, is established between the top of the metering chamber and the bottom of the withdrawal chamber so that there is practically no more surplus in the withdrawal chamber at the end of the transfer stage. Thus, during the pouring-out stage, practically only the metered amount is evacuated to the outside of the metering device.
The drawback with the aforesaid devices lies in the lack of accuracy of the metered amounts of liquid which are withdrawn and poured out, particularly when there is no longer a great deal of liquid in the container. In such a case in point, the hydrostatic pressure in the inverted position of the container may prove insufficient to completely fill the metering compartment.
This is particularly awkward when it is desired to pour out products of the liquid type which require precise metering.
Moreover, in known metering devices, it is necessary to prepare in advance a measure which is intended to be poured out and this remains stored, awaiting pouring-out. This may adversely affect metering in so far as a long period of non-use of the device or storage in a warm location, for example a greenhouse, leads to evaporation of at least a portion of the measure prepared in this way. In such a case, the actual amount of product delivered to a plant is unknown.